High power electrical motors are usually supplied by so-called medium voltage drives, which are used in various sectors to drive a variety of machines and processes. Such a medium voltage drive or an electrical converter may be characterised by the power flow between the load and a supplying grid and the energy storage provided by the DC link.
In a usual setup, the electrical converter absorbs power from a three-phase AC power source (the electrical grid), stores this power as energy in DC form using capacitors or inductors (i.e. in the DC link), and finally converts this stored energy back to AC form and drives an electric machine.
However, this power flow can be reversed for example when wind energy is harvested, i.e., the wind turbine converts the mechanical power to electric one, this in turn is rectified and stored, and finally the stored DC energy is inverted and fed back to the grid in an AC form. Alternating this power flow at least at the rectifier side or the inverter side may also be possible for short periods of time to allow extra controllability of the drive.
The AC current from the grid is converted into the DC current in the DC link via an active rectifier. The DC current from the DC link is converted into the AC current for the electrical machine via an inverter. These subunits of the electrical converter, the inverter at the machine side and the active rectifier unit on the grid side may be individually controlled so that the inverter delivers the required power to the electrical machine (hence the required torque at a given mechanical speed), and the active rectifier charges the DC link with the required power so that the stored energy remains close to a constant.
In case the capacitive components in the DC link are very large, disturbance on the machine side and the grid side may be ignored as they result in very little ripples in the stored energy. However, if the capacitive elements are undersized, then the utilized control method should be able to provide this constant energy property by design. For example, the controllers for inverter and active rectifier are separately designed with a power feedforward link between them. However, this may be not sufficient for large disturbances.
WO2015/028242 A2 relates to model predictive control of an electrical system comprising a rectifier, an inverter and an electrical load. The controller solves an optimization problem online by predicting a sequence of future states that is optimal with respect to a cost function, which considers the rectifier and the inverter.
U.S. Pat. No. 6,219,237 B1 describes a control method for a converter with an LC filter. This method is also based on comparing measured variables with outer loop control variables and by model predictive control. However, in U.S. Pat. No. 6,219,237 B1, a flux error is correct always with the same method, i.e. with a corrective flux, which is determined from filters and/or regulators.
EP 2 733 842 A1 discloses a further model predictive control method, in which an objective function is optimized iteratively.
A. Sapin et. al., “Modelling, Simulation and Test of a Three-level Voltage Source Inverter with Output Filter and Direct Torque Control”, IEEE 2003, Industrial Applications Conference, 38th IAS Annual Meeting, Oct. 12-16, 2003 relates to direct torque control of a specific inverter.